Flywheels typically comprise a relatively heavy mass, mounted on a shaft and arranged to rotate with the shaft. The use of flywheels in vehicles is known, for example in kinetic energy recovery systems for recovering kinetic energy from the movement of part or all of a vehicle and for subsequently returning that energy to the vehicle. Such systems are used in other applications, for example where energy is recovered from the boom of a working vehicle such as a loader. The kinetic or potential energy recovery is converted to kinetic energy of a flywheel.
The kinetic energy of a flywheel is directly proportional to the rotational inertia and the square of the angular velocity. A flywheel used for energy storage in a vehicle should achieve an optimum balance of mass, inertia and rotational speed. Consequently the faster the flywheel can be made to rotate, the smaller and lighter it will be for a given energy storage capacity.
High speed flywheels typically operate with maximum rotational speeds which are at least 15000 rpm. Such flywheels are usually contained within an enclosure which is at least partially evacuated, in order to reduce windage losses, i.e. energy losses caused by drag due to the movement of the flywheel through any fluid, e.g. air, in the enclosure. This helps to reduce the power consumption of the flywheel system, increasing the energy recoverable from the flywheel and also preventing the temperature of the flywheel from rising too high. This is particularly important where the flywheel is constructed from composite materials that include a resin, which are typically sensitive to high temperatures.
When a flywheel is contained within an evacuated enclosure, it is necessary to provide a seal between the housing and the shaft on which the flywheel is mounted, in order to allow the vacuum within the enclosure to be maintained. However, even with an effective seal, it is often necessary to “top-up” the vacuum by pumping any air or vaporised fluid (such as oil) that has leaked into the enclosure back out again, to maintain the very low pressure within the enclosure.
Creating the required vacuum level inside the enclosure can be challenging, particularly when creating it in the environment of a vehicle. Efficient vacuum pumps often require precision parts to achieve the desired vacuum pressures for flywheel operation. For example, a vacuum management system for a flywheel arrangement may include a precision pump in order to achieve the desired vacuum levels typically less than 4 mbar.
However the costs of such precision pumps tends to restrict their application to non-vehicle applications and makes vehicle-type applications undesirably expensive. Furthermore, in high mileage commercial vehicles and other challenging conditions, such as construction vehicles, trucks, distribution vehicles, buses and so on, durability and reliability are important factors. This further complicates the specifications of vacuum pumps for these applications.
Achieving the desirable vacuum levels within a short timeframe following start-up of the flywheel is also challenging because the ability to reduce pressure in the chamber becomes increasingly difficult as the chamber pressure approaches a true vacuum. Thus the pressure tends to fall asymptotically, approaching the final achievable near-vacuum pressure over a period of time. In other words, reducing the chamber pressure towards zero absolute pressure takes a considerable period of time.
This delay in achieving the desirable vacuum level means there is a period of time where the flywheel is not spun up or during which the rotation of the flywheel is allowed but results in undesirable windage losses. Alternatively, to mitigate this, the designer may install a high specification vacuum pump system. This will carry a penalty of reduced performance, or increased cost, respectively.
Storage and re-use of energy using flywheel storage systems may be used to reduce energy consumption or exhaust emissions of machines or vehicles. Enabling the flywheel system to operate optimally with low windage losses for the maximum time possible would enable the energy efficiency benefits of using the flywheel system to be maximised, because the flywheel chamber would be at a lower pressure for more of the time and so able to operate at its most efficient.
There is therefore a need for a flywheel system which is able to achieve and maintain optimally low flywheel chamber pressures for increased portions of the flywheel apparatus' operating time, without the undesirable need to use higher efficiency pumps which would add to the cost which would be prohibitive for vehicle-type applications. The present invention aims to provide a solution which achieves at least some of these aims.